1. Field of the Invention
The present invention relates to a stabilizing device for an optical modulator, and in particular, to a stabilizing device for an optical modulator which stabilizes an optical modulator which modulates inputted light by an optical waveguide and outputs the modulated light.
2. Description of the Related Art
A waveguide-type optical modulator, which modulates light propagated within a waveguide formed on a substrate, is known. Stabilizing the operating point of the waveguide-type optical modulator is an important technique. For example, when a Mach-Zehnder interferometer optical modulator is used, by setting and stabilizing the phase bias, which is the operating point, to .pi./2, optimal sensitivity and an optimal dynamic range characteristic can be achieved.
However, when the Mach-Zehnder interferometer optical modulator is used, the phase bias is set by the difference in the waveguide length when the modulating element is set. The actual value of the phase bias is several 100 nm, and manufacturing is not easy. Further, a problem arises in that slight changes in the ambient temperature or the stresses applied to the substrate or the like lead to the operating point (the phase bias) drifting. Namely, it is difficult to stabilize the operating point.
FIG. 1 illustrates the characteristic of an optical modulator at a time when the optical modulator is normal and at a time when the operating point drifts. In FIG. 1, the horizontal axis is the voltage applied to the electrode of the optical modulator, and the vertical axis is the intensity of the outputted light of the optical modulator. When the Mach-Zehnder interferometer optical modulator is used, the intensity of the outputted light varies as a cosine function of the applied voltage. At the normal characteristic (characteristic curve 100), the phase bias is set to .pi./2 and the operating point is the point of the characteristic curve 100 at which the slope is the greatest. Accordingly, the variation in the intensity of the outputted light with respect to the applied voltage due to an input signal 120 (i.e., a variation amount 104 in an output characteristic 102) is greatest, and the sensitivity is the highest.
However, if the operating point drifts from the normal characteristic (fluctuates by a phase bias fluctuation amount 130) to have the characteristic of characteristic curve 110, the variation in the intensity of the outputted light with respect to the applied voltage due to the same input signal 120 (i.e., a variation amount 114 in an output characteristic 112) is extremely small. As a result, some type of means is needed in order to compensate for the drift in the operating point.
In order to compensate for the drift in the operating point, drift compensating techniques have been proposed in which voltage corresponding to the fluctuation amount of the operating point is applied from the exterior (see Japanese Patent Applications Laid-Open (JP-A) Nos. 3-145623, 5-232412). In these drift compensating techniques, when light is modulated by using an optical modulator in optical communication, voltage corresponding to the fluctuation amount of the operating point is applied from the exterior so as to compensate for the drift.
FIG. 2 illustrates an optical modulator 140 in which drift in the operating point is compensated for. In the optical modulator 140, 1 is a laser diode, 2 is an LiNbO.sub.3 Mach-Zehnder interferometer optical modulator equipped with an electrode (hereinafter, "optical modulator"), 3 is a data signal input terminal, 4 is a modulator driving circuit, 5 is a coupling capacitor, 6 is a bias supplying circuit, 7 is a light branching device, 8 is an optical signal output terminal, 9 is a photodiode, 10 is a current voltage converter, 11 is an amplifier, 12 is a reference voltage source, 13 is a filter, and 14 is a metal wire which contacts the electrode.
A data signal inputted to input terminal 3 of the optical modulator 140 is inputted to the optical modulator 2 via the driving circuit 4 and the capacitor 5. At the optical modulator 2, the optical signal inputted from the laser diode 1 is intensity-modulated by an electrical signal inputted from the driving circuit 4. The data signal of the modified light is outputted from the output terminal 8.
The operating point of the optical modulator 2 at this time is controlled by the bias voltage supplied from the bias supplying circuit 6. More specifically, at the optical modulator 140, in order for voltage corresponding to the fluctuation amount of the operating point to be applied from the exterior, the metal wire 14 is connected to the electrode of the optical modulator 2. The operating point of the optical modulator 2 is controlled by an electric signal being applied to the wire 14. The control of the operating point is carried out by the light output waveform being monitored at the light branching device 7, the average value of the light output waveform being detected by the photodiode 9 and the current voltage converter 10, and the error voltage between the detected average value and reference value derived from the reference voltage source 12 being fedback as the bias voltage via the amplifier 11 and the filter 13. Namely, the deviation in the operating point is detected by monitoring the outputted light of the optical modulator 2, and the amount of deviation is fedback to voltage applied to the electrode of the optical modulator 2.
However, in a case in which a metal wire is connected to the optical modulator and an electrical signal is applied (as in the above-described case of the optical modulator 140), in the same way as an electric field probe, a waveguide-type modulator cannot be used in a light measuring device used in a sensor. Namely, the metal signal cable connected to the sensor disturbs the electromagnetic field to be measured, or an interference voltage is induced in the metal signal wire such that the signal/noise ratio decreases. Various problems, such as accurate measurement becoming difficult and the sensor becoming large, arise. As a result, the conditions of the environment in which the conventional optical modulator is used must be restricted by suppressing fluctuations in the ambient temperature by limiting the range of temperature in which the optical modulator is used, and by structuring the optical modulator such that loads such as stresses are not applied thereto, and the like.